Ultrasonic diagnostic apparatuses that produce an ultrasonic image as a diagnostic image of a subject transmit or receive ultrasonic waves to or from a subject via an ultrasonic probe, and reconstruct an ultrasonic image of the subject on the basis of a received signal sent from the ultrasonic probe. Known as the ultrasonic probe is a probe that has a plurality of oscillatory elements, which transforms ultrasonic waves into electric signals or vice versa, arrayed and that selects a plurality of driving oscillatory elements that defines an aperture through which ultrasonic waves are transmitted or received.
The resolution of an ultrasonic image depends on the width or diameter of an ultrasonic beam, which is transmitted or received by an ultrasonic probe, at a focal point P (hereinafter, generically, referred to as a beam width). In other words, when the beam width is narrowed in order to improve an azimuthal resolution, the image resolution improves.
For reduction of the beam width, a focusing technology is known. For example, assuming that the distance of a driving oscillatory element A, which is located in the center of the aperture, to a focal point P is a reference distance, a difference ΔL between the distance of any other driving oscillatory element B to the focal point P and the reference distance is calculated. The distance difference ΔL is proportional to a lag time spent to match the phases of ultrasonic waves. Focus data associated with the distance difference ΔL is prepared in advance. Transmission or reception of ultrasonic waves by the driving oscillatory elements A and B is caused to lag on the basis of the prepared focus data. Thus, the phases of ultrasonic waves are matched at the focus point P in order to reduce the beam width (refer to, for example, patent document 1 (JP-A-2004-274756)).
In the technology described in the patent document 1, the space (pitch) between adjoining driving oscillatory elements, such as, the driving oscillatory elements A and B is fixed to a certain value, and the driving oscillatory elements are not designated for each focal depth. Moreover, the distance difference ΔL varies depending on the depth position of the focal point P. Consequently, a method that prepares a plurality of focus data in association with different values of the distance difference ΔL is conceivable. However, according to this method, since the number of focus data is enormous, a memory capacity required to preserve the focus data increases, or anyway, a circuit scale increases.
Moreover, as a technology for reducing the beam width, focus control based on a so-called Fresnel zone to be created by bundling oscillatory elements is known. For example, a plurality of oscillatory elements are grouped into sets each including one oscillatory element or multiple adjoining oscillatory elements. A different phase is assigned in units of a set on the basis of a difference in a distance of each set to a focal point. Consequently, the phases of ultrasonic waves to be transmitted or received by the sets are matched at the focal point in order to thus reduce the beam width (refer to, for example, patent document 2 (U.S. Patent No. 2004/0160144)).
However, in the method described in the patent document 2, although a beam width is narrowed at a focal point, the beam width gradually expands along with an increase in a depth from the focal point. This may deteriorate image quality or degrade the resolution of an image expressing a deep region of a subject.
Moreover, when a focal point is set to a relatively deep position, the distance to the focal point gets enough larger than the distance between adjoining elements or adjoining sets. Consequently, the distances to the focal point from respective elements or sets are sometimes considered to be substantially the same. In this case, Fresnel-zone focus control based on a difference between the distances of sets to a focal point becomes hard to achieve. Consequently, there arises a fear that image quality may be deteriorated, or anyway, the resolution of an image expressing a deep region of a subject may be degraded.
An object of the present invention is to realize an ultrasonogaph capable of satisfactorily reducing a beam width while suppressing an increase in the number of focus data.